The present invention relates to a method according to the preamble of the appended claim 1 for converting signals in two-channel stereo format to become suitable to be played back using headphones. The invention also relates to a signal processing device according to the preamble of the appended claim 7 for carrying out said method.
Already for several decades the prevailing format for making music and other audio recordings and public broadcasts has been the well-known two-channel stereo format. The two-channel stereo format consists of two independent tracks or channels; the left (L) and the right channel, which are intended for playback using two separate loudspeaker units. Said channels are mixed and/or recorded and/or otherwise prepared to provide a desired spatial impression to a listener, who is positioned centrally in front of the two loudspeaker units spanning ideally 60 degrees with respect to the listener. When a two-channel stereo recording is listened through the left and right loudspeakers arranged in the above described manner, the listener experiences a spatial impression resembling the original sound scenery. In this spatial impression the listener is able to observe the direction of the different sound sources, and the listener also acquires a sensation of the distance of the different sound sources. In other words, when a two-channel stereo recording is listened, the sound sources seem to be located somewhere in front of the listener and inside the area substantially located between the left and the right loudspeaker unit.
Other audio recording formats are also known, which, instead of only two loudspeaker units, rely on the use of more than two loudspeaker units for the playback. For example, in a four channel stereo system two loudspeaker units are positioned in front of the listener: one to the left and one to the right, and two other loudspeaker units are positioned behind the listener: to the rear left and to the rear right, respectively. This allows to create a more detailed spatial impression of the sound scenery, where the sounds can be heard coming not only somewhere from the area located in front of the listener, but also from behind, or directly from the side of the listener. Such multichannel playback systems are nowadays commonly used for example in movie theatres. Recordings for these multichannel systems can be prepared to have independent tracks for each separate channel, or the information of the channels in addition to a normal two-channel stereo format can also be coded into the left and right channel signals in a two-channel stereo format recording. In the latter case a special decoder is required during the playback to extract the signals for example for the rear left and rear right channels.
Further, some special methods are known in order to prepare recordings, which are specially intended to be listened through headphones. These include, for example, binaural recordings that are made of recording signals corresponding to the pressure signals that would be captured by the eardrums of a human listener in a real listening situation. Such recordings can be made for example by using a dummy-head, which is an artificial head equipped with two microphones replacing the two human ears. When a high-quality binaural recording is listened through headphones, the listener experiences the original, detailed three-dimensional sound image of the recording situation.
The present invention is however mainly related to such two-channel stereo recordings, broadcasts or similar audio material, which have been mixed and/or otherwise prepared to be listened through two loudspeaker units, which said units are intended to be positioned in the previously described manner with respect to the listener. Hereinbelow, the use of the short term xe2x80x9cstereoxe2x80x9d refers to aforementioned kind of two-channel stereo format, if anything else is not separately mentioned. The listening of audio material in such stereo format through two loudspeakers is hereinbelow shortly referred to as xe2x80x9cnatural listeningxe2x80x9d.
During the last decade portable personal stereo devices, such as portable tape- and CD-players, for example, have become increasingly popular. This development has, among other things, strongly increased the use of headphones in the listening of music recordings, radio broadcasts etc. However, the commercially available music recordings and other audio material are almost exclusively in the two-channel stereo format, and thus intended for playback over loudspeakers and not over headphones. Despite of this fact, it is common to the portable stereo devices, and also to other playback systems, that they do not make any attempt to compensate for the fact that stereo recordings are intended for playback over loudspeakers and not over headphones.
When a stereo recording is played back over loudspeakers in a natural listening situation, the sound emitted from the left loudspeaker is heard not only by the listener""s left ear but also by the right ear, and correspondingly the sound emitted from the right loudspeaker is heard both by the right and left ear. This condition is of primary importance for the generation of a hearing impression with a correct spatial feeling. In other words, this is important in order to generate a hearing impression in which the sounds seem to originate from a space or stage outside. When listening a stereo recording over headphones, the left channel is heard in the left ear only, and the right channel is heard in the right ear only. This causes the hearing impression to be both unnatural and tiresome to listen to, and the sound scenery or stage is contained entirely inside the listener""s head: the sound is not externalised as intended.
Prior art methods, that are intended for improving the sound quality of two-channel stereo recordings when presented over headphones, come mainly in the following two types.
The first type of methods is based on the emulation of a natural listening situation, in which situation the sound would normally be reproduced through loudspeakers. In other words, the stereo signals played back through the headphones are processed in order to create in the listener""s ears an impression of the sound coming from a pair of xe2x80x9cvirtual loudspeakersxe2x80x9d, and thus further resembling the listening to the real original sound sources. Methods belonging to this category are referred later in this text as xe2x80x9cvirtual loudspeaker methodsxe2x80x9d.
The second type of methods is not based on attempting to create an accurate natural listening or natural sound scenery at all, but they rely on methods such as adding reverberation, boosting certain frequencies, or boosting simply the channel difference signal (L minus R). These methods have been empirically found to somewhat improve the hearing impression. Later in this text methods belonging to this category are referred as xe2x80x9cequalizersxe2x80x9d or xe2x80x9cadvanced equalizersxe2x80x9d.
In the following, the virtual loudspeaker method and the methods based on different types of equalizers are discussed in somewhat more detail.
If sound is emitted from a loudspeaker positioned for example to the left side of the listener, it is possible to determine the sound pressures created at the listener""s left and right ear. Comparing the loudspeaker input signal to the sound pressure signals observed at the listener""s left and right ear, it is possible to model the behaviour of the acoustic path that transfers the sound to the listener""s ears. When this is performed separately for both the left and right channels, it is further possible to realize signal filters, which can be used to process the loudspeaker input signals according to the behaviour of said acoustic paths. By processing the original signals using such filters, and playing back the filtered signals through headphones, ideally same sound pressures are reproduced at the listener""s ears as in the case of listening the original signals through loudspeakers. The above described virtual loudspeaker method is thus, at least in theory, a scientifically justified and credible method to emulate the natural listening conditions.
Each of the acoustic paths is made up of three main components: the radiation characteristics of the sound sources (such as a pair of loudspeakers), the influence of the acoustic environment (which causes early reflections from nearby surfaces and late reverberation), and the presence of the receiver (a human listener) in the sound field. The loudspeaker is usually not modelled explicitly, rather it is assumed to have a flat magnitude response and an omni-directional radiation pattern. The reflections from the acoustic environment are used by the listener to form an impression of the surroundings, and by modelling the early reflections [U.S. Pat. Nos. 5,371,799; 5,502,747; 5,809,149] and the late reverberation [U.S. Pat. Nos. 5,371,799; 5,502,747; 5,802,180; 5,809,149; 5,812,674], it is possible to give the listener the impression of being in an enclosed space. However, when using the given prior art methods this cannot be achieved without making a noticeable and negative change to the overall sound quality.
The effect of the receiver on the incoming sound waves, and in particular the effect of the human head and pinna (outer ear, earlobe), has been studied intensively by the research community for several decades. An acoustic path which includes a realistic modelling of the listener""s head, and possibly the listener""s torso and/or pinna, is usually referred to as a head-related transfer function (HRTF). HRTFs are usually measured on so-called dummy-heads under anechoic conditions, and it is common practice to equalize, i.e. to correct the raw measured data for the response of the transducer chain, which typically consists of an amplifier, a loudspeaker, a microphone, and some data acquisition equipment. The HRTF to the ear closest to the loudspeaker is referred to as the ipsilateral HRTF, whereas the HRTF to the other ear further away from the loudspeaker is referred to as the contralateral HRTF.
The human auditory system combines, and compares the sounds filtered by the ipsilateral and contralateral HRTFs for the purpose of localising a source of sound. It is a generally accepted fact that the auditory system uses different mechanisms to localise sound sources at low- and high frequencies. At frequencies below approximately 1 kHz, the acoustical wavelength is relatively long compared to the size of the listener""s head, and this causes an interaural phase difference to take place between the sound waves originating from a sound source (loudspeaker) and arriving to the listener""s two ears. Said interaural phase difference can be translated into an interaural time difference (ITD), which in other words is the time delay between the sound arriving at the listener""s closest and furthest ear. For sound sources in the horizontal plane, a large ITD means that the source is to the side of the listener whereas a small ITD means that the source is almost directly in front of, or directly behind, the listener.
At frequencies above approximately 2 kHz the acoustical wavelength is shorter than the human head, and the head therefore casts an acoustic shadow that causes an interaural level difference (ILD) to take place between the sound waves originating from a sound source and arriving at the listener""s two ears. In other words, the sound pressures arriving at the listener""s closest and furthest ear are different. At frequencies above 5 kHz, the acoustical wavelength is so short that the pinna contributes to large variations in interaural level difference ILD as a function of both the frequency and the position of the sound source.
Thus, localisation of sound sources at low frequencies is mainly determined by interaural time difference ITD cues whereas localisation of sound sources at high frequencies is mainly determined by interaural level difference ILD cues.
Prior art systems that implement the virtual loudspeaker method over headphones attempt to include both low frequency ITD cues and high-frequency ILD cues, at least to the extent that ILD is not constant above 3 kHz. There are many ways in which this high-frequency variation can be extracted and implemented [U.S. Pat. Nos. 3,970,787; 5,596,644; 5,659,619; 5,802,180; 5,809,149; 5,371,799; and also WO 97/25834]. One system even exaggerates the ILD in order to achieve a more convincing spatial effect [EP 0966 179 A2].
In practice, the drawbacks of the aforementioned virtual loudspeaker-type methods concentrate on the amount of detail contained in an accurate model of the acoustic paths, and further on the difficulties in being able to accurately design and realize the necessary signal filters. Today such filters can best be realized using digital signal processing techniques (DSP). However, the dynamic range of the necessary digital filters is rather large, and this has the undesirable side-effect that the filters introduce unwanted colouration of the reproduced sound. This colouration of the sound takes place especially at the higher frequencies, and it is particularly noticeable on high-fidelity recordings.
Methods that fall into categories of xe2x80x9cequalizersxe2x80x9d or xe2x80x9cadvanced equalizersxe2x80x9d cannot be considered to be so-called spatial enhancers in the strict sense of this definition, since they do not succeed in really externalising any part of the sound scenery. The basic idea of boosting the channel difference signal (L minus R channel) in a two-channel stereo format is based on the observation that the difference signal seems to contain more spatial information than the channel sum signal (L plus R). When headphones are used, the effect of increasing the level of the channel difference signal makes the sound sources at right and left to become more audible, whereas the sound sources near the centre are essentially unaffected. Thus, the sound components that are at the extreme left and extreme right on the sound scenery or stage are effectively made louder, but spatially they still remain at the same locations. However, if the effect boosts the overall sound level by a couple of decibels when it is switched on, it will sound like an improvement. In fact, an increase in the overall sound level will be usually interpreted by the listener as an improvement in the quality of the sound, irrespective of the method by means of which it was exactly accomplished. Most of the xe2x80x9cspatializerxe2x80x9d or xe2x80x9cexpanderxe2x80x9d functions that can be found today for example in tape players, CD-players or PC sound cards, can be considered as kind of advanced equalizers affecting the level of the channel difference signal [U.S. Pat. No. 4,748,669].
A known method is also to use a simple low-frequency boost, which is an effective method especially when used together with headphones. This is because headphones are much less efficient in reproducing low frequencies than loudspeakers. A low-frequency boost helps to restore the spectral frequency balance of the recording in playback, but no spatial enhancement can be achieved.
It is also known, that by adding reverberation to the stereo signals it is possible to give a listener an impression somewhat similar to the one experienced when listening music in a room or other similar closed space. It is well known that the ratio between direct sound and reflected, reverberated sound affects the human sensation of how far the sound source is experienced to be. The more reverberation, the farther away the sound source seems to be. However, high-quality, high-fidelity recordings already contain the correct amount of reverberation, and thus adding even more reverberation will degrade the result, usually giving an impression that the recording was performed in a basement or in a bathroom.
The main purpose of the present invention is to produce a novel and simple method for converting two-channel stereo format signals to become suitable to be played back using headphones. The present invention is based on a virtual loudspeaker-type approach and is thus capable of externalising the sounds so that the listener experiences the sound scenery or stage to be located outside his/her head in a manner similar to a natural listening situation. The aforementioned effect attained by using the method according to the invention is later in this text referred to as xe2x80x9cstereo wideningxe2x80x9d.
To attain this purpose, the method according to the invention is primarily characterized in what will be presented in the characterizing part of the independent claim 1.
Furthermore, it is the purpose of this invention to attain a signal processing device which implements the method according to the invention. The signal processing device according to the invention is primarily characterized in what will be presented in the characterizing part of the independent claim 7.
The other dependent claims present some preferred embodiments of the invention.
The basic idea behind the present invention is that it does not rely on detailed modelling of interaural level difference ILD cues, especially the high-frequency ILD cues; rather it omits excessive detail in order to preserve the sound quality. This is achieved by associating the high frequency ILD with a substantially constant value (equal for both channels L and R) above a certain frequency limit fHIGH, and also by associating the low frequency ILD with an another substantially constant value below a certain frequency limit fLOW.
In addition, the invention further sets the magnitude responses of the ipsilateral and contralateral HRTFs in such a way that their sum remains substantially constant as a function of frequency. Hereinbelow this is referred to as xe2x80x9cbalancingxe2x80x9d and it is different from prior art methods, including the ones described in WO 98/20707 and U.S. Pat. No. 5,371,799 which manipulate the contralateral HRTF only while maintaining a substantially flat magnitude response of the ipsilateral HRTF over the entire frequency range.
The method and device according to the invention are significantly more advantageous than prior art methods and devices in avoiding/minimizing unwanted and unpleasant colouration of the reproduced sound in the case of high-quality and high-fidelity audio material. In addition, the method according to the invention requires only a modest amount of computational power, being thus especially suitable to be implemented in different types of portable devices. The stereo widening effect according to the invention can be implemented efficiently by using fixed-point arithmetic digital signal processing by a specific filter structure.
An considerable advantage of the present invention is that it does not degrade the excellent sound quality available today from digital sound sources as for example CompactDisk players, MiniDisk players, MP3-players and digital broadcasting techniques. The processing scheme according to the invention is also sufficiently simple to run in real-time on a portable device, because it can be implemented at modest computational expense using fixed-point arithmetic.
When used in connection with the method according to the invention, compared to the sound reproduction via loudspeakers, headphone reproduction has the advantage of not depending on the characteristics of the acoustical environment, or on the position of the listener in that environment. The acoustics of a car cabin, for example, is very different from the acoustics of a living room, and the listener""s position relative to the loudspeakers is also different, and not necessarily ideal in these two situations. Headphones, however, sound consistently the same regardless of the acoustic environment, and further, if the type and characteristics of headphones are known in advance, it is possible to design a system which gives good sound reproduction in all situations. Furthermore, the capabilities of the modern high-quality and high-fidelity digital recording and playback facilities back up these possibilities well.
The preferred embodiments of the invention and their benefits will become more apparent to a person skilled in the art through the description hereinbelow, and also through the appended claims.